The present invention relates to a formed strip such as a roll formed strip, and a roll forming method or system.
In particular, the present invention relates to a stage for forming tubes in a production system for producing heat exchangers for air conditioners, including stages for producing header pipes by press working to form a flat sheet or plate to a cylindrical shape, for producing U-shaped holders by roll forming from a metal strip, for inserting, in each holder, an inner fin formed by press forming or roll forming, for forming tubes, such as heat exchanger tubes, from each holder, for forming corrugated outer fins by a corrugate cutter, and for assembling heat exchanger tubes, outer fins, and header pipes.
Tubes for conveying a fluid such as refrigerant is widely used in heat exchange devices such as a condenser of a refrigeration cycle for a motor vehicle, an evaporator and a radiator. In one production method, a refrigerant tube is formed by bending a flat metal strip.
When a metal tube is formed by applying pressure from both sides on a V-shaped metal strip until a V-shaped cross section is converted into a closed elongate cross section, the closure and joining of both edges of the open V-shaped cross section into the closed cross section is not easy because of undesired springback due to compressive stresses on the inner side of the curved portion of the metal strip and tensile stresses on the outer side. Moreover, springback tends to degrade the accuracy of the U-shaped or V-shaped cross section formed by bending operation.
It is therefore an object of t e present invention to provide a formed strip which can e made securely into a tube with no or little influence of springback.
It is another object of the present invention to provide a bending method or system which can bend a strip accurately with no or little influence of springback
It is still another object of the present invention to provide a method or system for forming flanged strips of different sizes in a manner reducing the production cost.
According to a first aspect of the present invention, a V-shaped metal strip having a V-shaped cross section for forming a metal tube having an closed elongated cross section comprises a U-shaped portion having a U-shaped curved inside surface and a U-shaped curved outside surface. Only the inside curved surface is formed with a plurality of fold lines extending in a longitudinal direction of the V-shaped metal strip.
According to a second aspect of the present invention, a bending method of bending a metal strip from a flat shape into a folded shape having an approximately U-shaped cross section comprises a preceding bending step and a following bending step.
The preceding bending step is for compressing the metal strip between a wider concave roll having a circumferential rim formed with a wider V-shaped groove defined by left and right sloping surfaces forming a wider spread angle and a wider convex roll having a circumferential rim formed with a wider ridge projecting toward a middle of the V-shaped groove between the left and right sloping surfaces and having a left edge for pressing the metal strip at a left preceding pressing point against the left sloping surface and a right edge for pressing the metal strip at a right preceding pressing point against the right sloping surface.
The following bending step is for compressing the metal strip between a narrower concave roll having a circumferential rim formed with a narrower V-shaped groove defined by left and right sloping surfaces forming a narrower spread angle smaller than the wider spread angle of the preceding bending step and a narrower convex roll having a circumferential rim formed with a narrower ridge projecting toward a middle of the narrower V-shaped groove between the left and right sloping surfaces and having a left edge for pressing the metal strip at a right following pressing point against the left sloping surface and a right edge for pressing the metal strip at a left following pressing point against the right sloping surface, the left and right following pressing points being located between the left and right preceding pressing points.
A forming or bending method for forming a folded metal strip according to the second aspect of the invention may comprise a sequence of bending steps, each for applying two parallel pushing forces on a metal strip at two application points toward a V-shaped groove forming an angle. The distance between the two application points and the angle of the V-shaped groove are decreased step by step in the sequence of the bending steps.
According to a third aspect of the present invention, a roll forming system comprises first and second roll.
The first roll includes a main roll section having a main roll surface for pressing a main strip section of a strip and a side roll section raised from main roll surface of the first roll and having a side roll surface for forming a flange in the strip.
The second roll includes a main roll section having a main roll surface for compressing the main strip section of the strip between the main roll surfaces of the first and second rolls, a side roll section depressed below the main roll surface of the second roll and having a first side roll surface for forming the flange in the strip with the side roll surface of the first roll when the first roll has the side roll section at a first axial location confronting the first side roll surface of the side roll section of the second roll, and an annular groove formed axially between the main roll section of the second roll and the side roll section of the second roll, for forming the flange in the strip with the side roll surface of the first roll when the first roll has the side roll section at a second location for fitting in the annular groove of the second roll.